1. Field of the Invention
The present invention relates to a fabrication method of Surface Acoustic Wave (SAW) devices, more particularly, which can fabricate a multi-band SAW device having electrodes of different thickness formed on a single substrate.
2. Description of the Related Art
At the present, SAW devices are fabricated as a device that has at least two SAW filers formed on a single substrate in order to obtain dual band or multi-band filter characteristics. A multi-band SAW device necessarily has electrodes of different mass (which is referred to as mass-loading effect) in order to have filtering ability in various frequency bands. For the purpose of this, there are schemes of providing electrodes of different thickness for various frequency bands.
As a conventional approach, Japanese Patent Publication No. 10-190390, filed on Dec. 25, 1996, Murata Mfg. Co. Ltd., discloses a SAW device fabrication method which forms electrodes of different thickness on a single substrate via photolithography and etching. FIGS. 1A to 1E are stepwise sectional views illustrating a fabrication method of SAW devices for dual band filters disclosed in this document.
According to the above document, as shown in FIG. 1A, a conductive layer 2 which is to form itself into a first electrode of a first SAW filter is deposited on a piezoelectric ceramic substrate 1, and a photoresist pattern 5 in use for forming the first electrode is provided on the conductive layer 2 of the first SAW filter. Exposed portions of the conductive layer 2 is selectively removed with etchant, as shown in FIG. 1B, to provide the first electrode 2a of the first SAW filter 10a on a first region of the piezoelectric ceramic substrate 1. Next, a photoresist pattern 6 in use for forming a second electrode of a second SAW filter is provided on the piezoelectric ceramic substrate 1 as shown in FIG. 1C, and a conductive layer 4 of a thickness corresponding to the second electrode is deposited on the resultant structure as shown in FIG. 1D. Then, a portion 4b of the conductive layer 4 is lifted off along with the photoresist pattern 6 to provide the second electrode 4a of the second SAW filter 10b on a second region of the piezoelectric ceramic substrate 1.
On this wise, Japanese Patent Publication No. 10-190390 forms the first and second SAW filter electrodes 2a and 4a of different thickness on the same piezoelectric ceramic substrate 1 via photolithography and etching to produce a SAW device 10 as a dual band filter.
However, this fabrication method may bring undesirable problems owing to photlithography and lift-off procedures. First of all, the photoresist patterning step can rarely produce a clean side profile in the pattern owing to restrictions of conventional photolithography. This problem degrades the side profile of the photoresist pattern, and thus gives as a drawback a more uneven side profile to the second electrode 4a obtained through the lift-off procedure than the side of the first electrode 2a obtained through the step of FIG. 1B, thereby deteriorating the characteristics of the second SAW filter 10b. 
Furthermore, there is a problem in that the electrodes become highly corrodible because they are exposed to a high humidity environment of for example stripping solution during the lift-off procedure. In the meantime, a multi-band SAW device requires at least two lift-off procedures in order to produce at least three SAW filters of different electrode thickness. The repeated lift-off procedures worsen the above problems while excessively complicating the entire process.